Catalyst and process using the catalyst

ABSTRACT

A new chromium-containing fluorination catalyst is described. The catalyst comprises an amount of zinc that promotes activity. The zinc is contained in aggregates which have a size across their largest dimension of up to 1 micron. The aggregates are distributed throughout at least the surface region of the catalyst and greater than 40 weight % of the aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in those aggregates.

This invention relates to a chromium-containing fluorination catalyst and to a process for the production of fluorinated hydrocarbons that uses the catalyst. More particularly, the invention relates to a zinc promoted, chromium-containing fluorination catalyst and to a process for the production of a fluorinated hydrocarbon in which a hydrocarbon or halogenated hydrocarbon is reacted with hydrogen fluoride in the presence of the catalyst.

The production of fluorinated hydrocarbons, which may also contain halogen atoms other than fluorine, by the catalysed vapour-phase fluorination of hydrocarbons or halogenated hydrocarbons with hydrogen fluoride is well known and numerous catalysts have been proposed for use in such processes. Catalysts containing and typically based on chromium, and in particular chromia, are frequently employed in the known processes. Thus, for example, chromia or a halogenated chromia may be used in the vapour-phase reaction of trichloroethylene with hydrogen fluoride to produce 1-chloro-2,2,2-trifluoroethane as described in GB-1,307,224 and in the vapour-phase reaction of 1-chloro-2,2,2-trifluoroethane with hydrogen fluoride to produce 1,1,1,2-tetrafluoroethane as described in GB-1,589,924. The same catalyst may also be used for the fluorination of chlorodifluoroethylene to 1-chloro-2,2,2-trifluoroethane, for example in a process for the removal of chlorodifluoroethylene impurity from 1,1,1,2-tetrafluoro ethane as also described in GB-1,589,924.

EP-A-0502605 discloses a chromium-containing fluorination catalyst which comprises an activity-promoting amount of zinc or a compound of zinc. The catalyst can be used in a process for preparing 1,1,1,2-tetrafluoroethane in which 1-chloro-2,2,2-trifluoroethane is reacted with hydrogen fluoride in the presence of the catalyst to produce the 1,1,1,2-tetrafluoroethane. The 1-chloro-2,2,2-trifluoroethane may be prepared by reacting trichloroethylene with hydrogen fluoride in the presence of the same catalyst.

Manufacturers of fluorinated hydrocarbons are always seeking improved catalysts for use in the manufacture of those compounds. It has now been found that the stability of chromium-containing catalysts incorporating controlled amounts of zinc may be improved if the distribution of zinc in the catalyst meets certain criteria.

According to the present invention there is provided a chromium-containing fluorination catalyst which comprises an amount of zinc, said zinc being contained in aggregates which have a size across their largest dimension of up to 1 micron and which are distributed throughout at least the surface region of the catalyst and wherein greater than 40 weight % of the aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in those aggregates.

The present invention also provides a process for the production of fluorinated hydrocarbons which comprises reacting a hydrocarbon or a halogenated hydrocarbon with hydrogen fluoride in the vapour phase in the presence of a fluorination catalyst as herein defined.

The zinc in the chromium-containing fluorination catalysts of the present invention is contained in aggregates that are evenly distributed throughout at least the surface region of the catalyst. By the surface region of the catalyst, we are intending to refer to that portion of the catalyst that will contact the hydrogen fluoride and organic reactants during use. The surface of a catalyst is generally that region where the coordination or valency of the atoms is not satisfied when compared to the bulk material. Preferably, the zinc-containing aggregates are evenly distributed throughout the entire catalyst bulk. The aggregates have a size across their largest dimension of up to 1 micron (1 μm), preferably in the range of from 20 nm to 1 μm, and greater than 40 weight %, preferably greater than 50 weight %, more preferably greater than 60 weight % and especially greater 70 weight % of the aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in those aggregates. In a preferred embodiment, greater than 80 weight %, more preferably greater than 85 weight %, and especially greater than 90 weight % of the aggregates contain a concentration of zinc that is within ±2 weight % of the modal concentration of zinc in those aggregates.

The modal concentration of zinc in the aggregates is that concentration of zinc that occurs most frequently in the aggregates expressed as a whole number.

By ‘evenly distributed’ we include ‘substantially evenly distributed’ and mean that the number or density of zinc-containing aggregates in each region of the catalyst surface or the catalyst bulk, where the zinc is dispersed throughout the entire catalyst, is substantially the same. For example, where the aggregates are only present at the catalyst surface, the number of aggregates in each square millimeter of the catalyst surface is within ±2% of the mean number of aggregates per square millimeter of the catalyst surface. Where the zinc-containing aggregates are distributed throughout the entire catalyst bulk, the number of aggregates in each square millimeter of the catalyst bulk is within ±2% of the mean number of aggregates per square millimeter of the catalyst bulk.

In a preferred embodiment, the chromium-containing fluorination catalysts of the invention comprise one or more compounds selected from the chromium oxides, the chromium fluorides, fluorinated chromium oxides and the chromium oxyfluorides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional bar graphical illustration of zinc distribution across the surface of a typical sample of Catalyst A, identifying frequency of aggregate 0.5-1.0 micron in size and sub-volumes 20-30 nm in diameter relative to the percentage of ZnO.

FIG. 2 is a three dimensional bar graphical illustration of zinc distribution across the surface of a typical sample of Catalyst B, identifying frequency of aggregate 0.5-1.0 micron in size and sub-volumes 20-30 nm in diameter relative to the percentage of ZnO.

FIG. 3 is a linear graphical comparison of the stability between Catalysts A and B illustrated by the temperature required to achieve 10% 133a to 134a conversion over a series of simulated operating cycles.

The chromium compounds which make up the chromium-containing catalyst of the invention can contain chromium in any of its usual oxidation states, namely chromium (II), chromium (III) and chromium (VI). However, the bulk or perhaps all of the chromium compounds in the catalysts will usually be based on chromium (III), although in a preferred embodiment from 0.1 to 8.0% by weight of the chromium based on the total weight of chromium in the catalyst will be present as chromium (VI). The performance, particularly the activity and stability, of zinc promoted chromium-containing catalysts can be improved if some of the chromium in the catalyst is present as chromium (VI)

Chromium (III) may typically comprise from 92.0 to 100% by weight, preferably from 94.0 to 100% by weight and particularly from 96.0 to 100% by weight of the total weight of chromium in the catalyst. Where chromium (VI) is also present, the catalyst of the invention typically comprises from 92.0 to 99.9% by weight, preferably from 94.0 to 99.9% by weight, e.g. from 95.0 to 99.5% by weight, particularly from 96.0 to 99.5% by weight and especially from 96.0 to 99.0% by weight, e.g. from 98.0 to 99.0% by weight of chromium (III) and from 0.1 to 8.0% by weight, preferably from 0.1 to 6.0% by weight, e.g. from 0.5 to 5.0% by weight, particularly from 0.5 to 4.0% by weight and especially from 1.0 to 4.0% by weight, e.g. from 1.0 to 2.0% by weight of chromium (VI) based on the total weight of chromium in the catalyst. As all the chromium is usually present as chromium compounds, the percentages quoted above will also normally define the amounts of chromium (III) and chromium (VI) compounds in the catalyst based on the total weight of chromium compounds.

Chromium (III) compounds that may be present in the chromium-containing catalyst of the invention include compounds selected from the group consisting of chromium (III) hydroxide, chromia (i.e. chromium (III) oxide), chromium (III) fluoride, fluorinated chromia and chromium (III) oxyfluorides. Chromium (VI) compounds that may be present in the catalyst include compounds selected from the group consisting of chromium (VI) oxide, chromic acid, fluorinated chromium (VI) oxide, chromium (VI) oxyfluorides and chromyl fluoride. The catalyst preferably contains one or more chromium (III) compounds and one or more chromium (VI) compounds selected from the above groups of compounds. The precise constitution of the catalyst will depend, inter alia, on the methods used for its preparation and whether the composition of the catalyst is measured pre- or post-fluorination.

Before the catalyst of the present invention is used in a fluorination process or before it is subjected to a fluorination pre-treatment, a significant proportion of the chromium, e.g. in excess of 50.0 weight % and more typically in excess of 75.0 weight % based on the total weight of chromium in the catalyst, is preferably present in the catalyst as chromium oxides, including chromia and preferably chromium (VI) oxide. It may also contain an amount of chromium hydroxides, including chromium (III) hydroxide and perhaps chromium (VI) hydroxide. The amounts of the chromium (III) oxides and hydroxides combined and, when present, the amounts of the chromium (VI) oxides and hydroxides combined are preferably as discussed above for the chromium (III) and chromium (VI) compounds generally. A preferred chromium-containing catalyst, pre-fluorination, has a molar ratio of chromium (III) to oxygen to hydroxyl species (Cr (III):O:OH) in the range of from 1:0.5:2 to 1:1.5:0, preferably in the range of from 1:1:1 to 1:1.5:0. This ratio can be readily determined using thermogravimetric analysis. In one particular embodiment, the chromium-containing catalyst, pre-fluorination, has a molar ratio of chromium (III) to oxygen to hydroxyl species (Cr (III):O:OH) in the range of from 1:0.5:2 to 1:n:m, preferably in the range of from 1:1:1 to 1:n:m, where n is less than 1.5, m is greater than zero and 2n+m=3.0.

When the catalyst is used in a fluorination process, or when it is subjected to a fluorination pre-treatment to be described hereinafter, a proportion of the chromium oxides in the catalyst and any chromium hydroxides that may be present will be converted to chromium fluorides, fluorinated chromium oxides and/or chromium oxyfluorides.

The zinc/chromia catalysts used in the present invention may be amorphous. By this we mean that the catalyst does not demonstrate substantial crystalline characteristics when analysed, for example, by X-ray diffraction.

Alternatively, the catalysts may be partially crystalline. By this we mean that from 0.1 to 50% by weight of the catalyst is in the form of one or more crystalline compounds of chromium and/or one or more crystalline compounds of zinc. If a partially crystalline catalyst is used, it preferably contains from 0.2 to 25% by weight, more preferably from 0.3 to 10% by weight, and particularly from 0.4 to 5% by weight of one or more crystalline compounds of chromium and/or one or more crystalline compounds of zinc.

The amount of crystalline material in the catalysts of the invention can be determined by any suitable method known in the art. Suitable methods include X-ray diffraction (XRD). When XRD diffraction is used, the amount of crystalline material, such as the amount of crystalline chromium oxide, can be determined with reference to a known amount of graphite present in the catalyst (e.g. graphite used in producing catalyst pellets) or, more preferably, by comparison of the intensity of the XRD patterns of the sample materials with reference materials prepared by suitable internationally recognised bodies, for example NIST (National Institute of Standards and Technology), that contain a known amount of a crystalline material.

The zinc is usually present in the catalyst as a zinc compound and may be present in or on the chromium-containing catalyst, that is the zinc or zinc compound may be incorporated in the chromium-containing catalyst or it may be supported on the surface of the catalyst, depending at least to some extent upon the particular method employed for preparing the catalyst. If the zinc is incorporated throughout the chromium-containing catalyst, as is preferred, then it should be substantially evenly distributed throughout the catalyst bulk.

The zinc is typically present in the catalyst in an amount of from 0.5 to 25% by weight based on the total weight of the catalyst. The amount of zinc is important, because at the right levels it will promote the activity of the chromium-containing catalyst. Too much zinc, on the other hand, may result in a decrease rather than an increase in catalyst activity.

The amount of zinc which will promote catalyst activity and produce a catalyst having an activity that is greater than the chromium-containing catalyst alone depends, at least to some extent, on the surface area of the catalyst and whether the zinc is incorporated throughout the catalyst bulk or just supported on its surface. Generally, the larger the working surface area of the catalyst, the greater is the amount of zinc which will be required to promote catalyst activity. Furthermore, catalysts containing zinc incorporated throughout their bulk, i.e. at surface and non-surface locations, will tend to require larger amounts of zinc than those catalysts which only have zinc on their surface.

By way of example, in the case of a catalyst where the zinc is introduced by impregnation to reside predominantly at the catalyst surface, activity promoting amounts of zinc for a chromium-containing catalyst having a working surface area of between 20 and 50 m²/g are usually in the range of from about 0.5% to about 6.0% by weight based on the total weight of the catalyst, preferably in the range of from about 1.0% to about 5.0% by weight and especially in the range of from about 2.0% to about 4.0% by weight.

However, for catalysts having larger working surface areas, for example greater than 100 m²/g, and comprising zinc distributed throughout the catalyst bulk, the zinc may be present in an amount of from 5.0% to 25.0% by weight based on the total weight of the catalyst, preferably in an amount of from 5.0 to 20.0% by weight and especially in an amount of from 5.0 to 10.0% by weight.

For catalysts having small working surface areas, i.e. less than 20 m²/g, for example about 5 m²/g, the amount of zinc may be as low as 0.5% to 1% by weight based on the total weight of the catalyst.

It should be understood that the amounts of zinc discussed above refer to the amount of zinc itself, whether present as elemental zinc or as a compound of zinc. Thus, where the zinc is present as a compound of zinc, as is usual, the amounts refer to the zinc provided by the zinc compound and not to the amount of the compound of zinc.

Preferred catalysts of the invention have a surface area in the range of from 20.0 to 300.0 m²/g, more preferably in the range of from 100 to 250 m²/g and particularly in the range of from 180 to 220 m²/g. When referring to the surface area of the catalyst, we are referring to the surface area prior to any fluorination treatment when measured by BET nitrogen isotherm (see, for example, G C Bond, Heterogeneous Catalysis—Principles and Applications 1987). These catalysts preferably comprise from 0.5 to 25.0% by weight, more preferably from 0.5 to 10.0% by weight and particularly from 1.0 to 6.0% by weight of zinc based on the total weight of the catalyst. The zinc can be distributed throughout the catalyst at surface and non-surface locations or just at the surface.

Although the amount of zinc which will promote catalyst activity will vary depending, inter alia, on the surface area of the catalyst, upon the distribution of zinc in the catalyst and upon the method that is used to prepare the catalyst, for any particular catalyst and catalyst preparation method, the amount of zinc that will promote catalyst activity is readily determined by routine experimentation using the above percentages as a guide.

The chromium-containing catalyst may also comprise metal oxides, fluorinated metal oxides, metal fluorides or metal oxyfluorides other than chromium oxides, fluorinated chromium oxides, chromium fluorides or chromium oxyfluorides. The additional metal oxide may, for example, be selected from alumina, magnesia and zirconia, and in particular magnesia and alumina, which during operation of the catalyst may be converted at least in part to aluminium fluoride and magnesium fluoride respectively.

If desired, the catalyst may also contain one or more metals other than zinc, for example nickel, cobalt or other divalent metals. Preferably, however, the chromium-containing catalyst will comprise just zinc, either as a metal but more typically as one or more zinc compounds.

The chromium-containing catalyst of the invention may also be supported on a catalyst support material such as activated carbon or alumina.

The zinc promoter may be introduced into and/or onto the chromium-containing catalyst in the form of a compound, for example a halide, oxyhalide, oxide or hydroxide, depending at least to some extent upon the catalyst preparation technique employed. When the zinc promoter is introduced by impregnating a chromium-containing catalyst, e.g. one containing one or more chromium (III) compounds and optionally one or more chromium (VI) compounds, with a zinc compound, the zinc compound is preferably a water-soluble salt, for example a halide, nitrate or carbonate, and is impregnated into the chromium-containing catalyst by contacting the catalyst with an aqueous solution or slurry of the zinc compound.

In an alternative method for preparing the catalyst of the invention, the hydroxides of zinc and chromium are co-precipitated and then converted to their oxides by calcination to prepare a mixed oxide catalyst.

If other metal oxides are to be included in the catalyst, such as alumina, then these can be introduced by co-precipitating the hydroxides of chromium and the other metal and then converting the hydroxides to their oxides by calcination to prepare a mixed oxide catalyst, e.g. of chromium and aluminium oxides such as chromia and alumina. Zinc can be introduced into the catalyst by impregnating the hydroxide or oxide mixture with an aqueous solution or dispersion of a zinc compound in the manner discussed above. Alternatively, zinc hydroxide can be co-precipitated with the hydroxides of chromium and the other metal and the three hydroxides then converted simultaneously to their oxides by calcination.

In a preferred embodiment, the catalysts of the present invention are prepared by adding zinc and chromium (III) salts to water and then co-precipitating the hydroxides of zinc and chromium (III) by adding a suitable inorganic hydroxide and preferably ammonium hydroxide to the aqueous salt solution. The co-precipitation is conducted under mixing conditions that will result in the zinc being evenly distributed throughout the catalyst. The mixture of zinc and chromium hydroxides is then collected, e.g. by filtration, washed and calcined to convert the hydroxides to their oxides. Any water soluble and stable salts of zinc and chromium can be used including the chlorides, carbonates and nitrates. Preferred salts of chromium include chromium nitrate and basic chromium nitrate (Cr(NO₃)₂.OH). A particularly suitable chromium salt is chromium (III) nitrate. A preferred zinc salt is zinc nitrate.

The washing process following collection of the mixed hydroxide precipitate can be important, because if the precipitate is prepared from a solution containing a nitrate salt then any nitrate that remains following the washing process can act as an oxidant to generate chromium (VI) from chromium (III) during the calcination process. The presence of a small amount of chromium (VI) in the catalyst can further improve catalyst activity and stability. More thorough washing of the collected precipitate, e.g. by repeated washing using fresh batches of washing liquor, will tend to reduce the residual nitrate levels and hence the amount of nitrate that is available to oxidise the chromium (III) during the calcination step. Furthermore, the nature of the washing medium can influence the efficacy with which nitrate contained in the mixed hydroxide precipitate is removed. For example, washing with an aqueous ammonia solution is more effective at removing the nitrate than water alone. Thus, if the mixed hydroxide precipitate is prepared from an aqueous solution containing chromium (III) and/or zinc nitrate, it is possible to control the level of chromium (VI) in the catalyst following calcination by exercising control over the washing process, which in turn will affect the residual level of nitrate in the precipitate that is available to oxidise the chromium (III).

Where a calcination step is employed in the production of the catalysts of the invention, as is preferred, it typically involves heating the precursor catalyst material at a temperature in the range of from 300 to 450° C., more preferably in the range of from 300 to 400° C., for example around 350° C. The calcination may be conducted in an inert atmosphere, e.g. of nitrogen, or it may be conducted in air or in an atmosphere comprising air or oxygen in mixture with an inert gas such as nitrogen.

The calcination temperature that is used can also influence the level of chromium (VI) in the final catalyst. For example, if the catalyst is prepared by calcining a mixed hydroxide precipitate prepared from an aqueous solution containing chromium (III) and/or zinc nitrate, then for a given level of residual nitrate following washing, higher calcination temperatures will tend to result in more of the chromium (III) being oxidised to chromium (VI).

Another convenient way of generating a desired level of chromium (VI) compounds in the catalyst is by introducing a controlled amount of air into the calcination step to oxidise the requisite proportion of chromium (III) to chromium (VI). Here again, the calcination temperature that is used can also influence the level of chromium (VI) in the final catalyst, with higher calcination temperatures tending to encourage greater oxidation of the chromium (III) for a given level of air.

The fluorination catalyst will usually be subjected to a fluorination treatment by heating in the presence of hydrogen fluoride, and optionally an inert diluent, prior to being used in the catalysis of fluorination reactions. A typical fluorination treatment comprises heating the catalyst in the presence of hydrogen fluoride at a temperature in the range of from 250 to 450° C., more preferably in the range of from 300 to 380° C. and particularly in the range of from 350 to 380° C. In a preferred embodiment, the fluorination treatment is conducted by contacting the fluorination catalyst with a mixture of hydrogen fluoride and nitrogen. Conveniently, the treatment is conducted in the reactor in which the subsequent fluorination process is to be conducted by passing the hydrogen fluoride or hydrogen fluoride/nitrogen mixture through the reactor while it is heated.

Following the fluorination treatment, the working catalyst usually comprises at least a proportion of zinc fluoride in and/or on a fluorinated chromium-containing catalyst material comprising one or more fluorine-containing chromium (III) compounds and preferably a small amount of one or more fluorine-containing chromium (VI) compounds selected from the fluorinated chromium oxides, the chromium fluorides and the chromium oxyfluorides. Where the catalyst is a mixed oxide catalyst prepared by co-precipitation of chromium and zinc hydroxides followed by calcination to convert the hydroxides to their oxides, as is preferred, the fluorination treatment usually converts at least a proportion of the oxides to oxyfluorides and fluorides.

The catalyst may be in the form of pellets or granules of appropriate shape and size for use in a fixed bed or a fluidised bed. Conveniently the catalyst is in the form of cylindrically shaped pellets having a length and diameter in the range of from 1 to 6 mm, preferably in the range of from 2 to 4 mm, for example 3 mm.

After a period of use catalysing a fluorination reaction, the used catalyst may be regenerated or reactivated, for example by heating in air/nitrogen or air/hydrogen fluoride mixtures at a temperature of from about 300° C. to about 500° C. The regeneration or reactivation may be conducted periodically until the catalyst has reached the end of its useful lifetime. The catalyst may also be regenerated by passing chlorine through the reactor while heating the catalyst. Alternatively, the catalyst may be regenerated continuously while the process is being operated.

A further aspect of the present invention resides in the use of the zinc-promoted, chromium-containing catalyst in a fluorination process in which a hydrocarbon or halogenated hydrocarbon is reacted with hydrogen fluoride in the vapour-phase at elevated temperatures.

Accordingly, the present invention also provides a process for the production of fluorinated hydrocarbons which comprises reacting a hydrocarbon or a halogenated hydrocarbon with hydrogen fluoride at elevated temperature in the vapour phase in the presence of a fluorination catalyst as herein defined.

Alkenes and alkanes as well as their halogenated counterparts containing at least one chlorine atom may be fluorinated using hydrogen fluoride and the catalysts of the present invention. Examples of specific vapour phase fluorinations which may be effected are the production of 1,1,1,2-tetrafluoroethane from 1-chloro-2,2,2-trifluoroethane, the production of 1-chloro-2,2,2-trifluoroethane from trichloroethylene, the production of pentafluoroethane from dichlorotrifluoroethane, the production of dichlorotrifluoroethane, chlorotetrafluoroethane and/or pentafluoroethane from perchloroethylene and the conversion of 1-chloro-2,2-difluoroethylene to 1-chloro-2,2,2-trifluoroethane.

The fluorination conditions employed when reacting the hydrocarbon or halogenated hydrocarbon with hydrogen fluoride in the presence of the catalyst of the invention may be those known in the art for fluorination reactions that employ chromium-containing catalysts, for example atmospheric or super-atmospheric pressures and reactor temperatures in the range of from 180° C. to about 500° C. When referring to the reactor temperature, we are referring to the mean temperature within the catalyst bed. It will be appreciated that for an exothermic reaction, the inlet temperature will be lower than the mean temperature, and for an endothermic reaction, the inlet temperature will be greater than the mean. The precise conditions will depend, of course, upon the particular fluorination reaction being carried out.

In a preferred embodiment, the catalyst of the invention is used in a process for preparing 1,1,1,2-tetrafluoroethane which comprises reacting 1-chloro-2,2,2-trifluoroethane with hydrogen fluoride in the vapour phase at elevated temperatures in the presence of the catalyst. Reaction temperatures in the range of from 250 to 500° C. are typically employed, with reaction temperatures in the range of from 280 to 400° C. being preferred and reaction temperatures in the range of from 300 to 350° C. being especially preferred. The process may be carried out under atmospheric or super-atmospheric pressures. Pressures of from 0 to 30 barg are preferred whilst pressures of from 10 to 20 barg are especially preferred.

In a further preferred embodiment, the catalyst of the invention is used in a multi-step process for preparing 1,1,1,2-tetrafluoroethane which comprises reacting trichloroethylene with hydrogen fluoride in the presence of the catalyst to form 1-chloro-2,2,2-trifluoroethane. The 1-chloro-2,2,2-trifluoroethane is then reacted with further hydrogen fluoride in the presence of the catalyst to form the 1,1,1,2-tetrafluoroethane. The conversion of trichloroethylene to 1-chloro-2,2,2-trifluoroethane and the conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane may be conducted in discrete reaction zones of a single reactor, but they are preferably conducted in different reactors. Both reactions are conducted at elevated temperatures in the vapour phase.

The preferred pressure and temperature conditions for the conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane are as specified above.

For the conversion of trichloroethylene to 1-chloro-2,2,2-trifluoroethane, the process is typically conducted at a temperature in the range of from 180 to 300° C., preferably in the range of from 200 to 280° C. and particularly in the range of from 220 to 260° C. Atmospheric or super-atmospheric pressures may be employed in the process. Typically, the process is conducted at a pressure in the range of from 0 to 30 bar, preferably in the range of from 10 to 20 barg.

A particularly preferred embodiment of the above-described multi-step process for preparing 1,1,1,2-tetrafluoroethane from trichloroethylene comprises the steps of:

-   (A) in a first reaction zone reacting 1-chloro-2,2,2-trifluoroethane     with hydrogen fluoride in the vapour phase in the presence of a     fluorination catalyst of the invention at a temperature of from 250     to 450° C. so as to form a product mixture containing     1,1,1,2-tetrafluoroethane and hydrogen chloride together with     unreacted starting materials; -   (B) conveying the total product mixture of step (A) as well as     trichloroethylene and optionally further hydrogen fluoride to a     second reaction zone containing a fluorination catalyst of the     invention and in said second reaction zone reacting the     trichloroethylene with hydrogen fluoride in the vapour phase at 180     to 350° C. to form 1-chloro-2,2,2-trifluoroethane; -   (C) collecting from step (B) a product mixture comprising     1-chloro-2,2,2-trifluoroethane, 1,1,1,2-tetrafluoroethane and     hydrogen chloride; -   (D) treating the product of step (C) to recover     1,1,1,2-tetrafluoroethane and produce a composition comprising     1-chloro-2,2,2-trifluoroethane that is suitable for conveying to the     first reaction zone in step (A); -   (E) conveying the 1-chloro-2,2,2-trifluoroethane composition     obtained from step (D) optionally together with further hydrogen     fluoride to said first reaction zone; and -   (F) collecting 1,1,1,2-tetrafluoroethane recovered in step (D).

Although the process described above refers to first and second reaction zones, this should not be taken as limiting the process to a particular order. In chemical terms, trichloroethylene is first converted to 1-chloro-2,2,2-trifluoroethane and the 1-chloro-2,2,2-trifluoroethane is then subsequently converted to 1,1,1,2-tetrafluoroethane. Thus, the first reaction in the reaction sequence is the hydrofluorination of trichloroethylene to form 1-chloro-2,2,2-trifluoroethane.

The first and second reaction zones may be provided by first and second reactors or they may be discrete zones of a single reactor. Preferably, the first and second reaction zones are provided by first and second reactors.

At least the stoichiometric amount of hydrogen fluoride is usually employed in step (A) of the above process. Typically, from 1 to 10 moles of hydrogen fluoride and preferably from 1 to 6 moles of hydrogen fluoride are used per mole of 1-chloro-2,2,2-trifluoroethane. Accordingly, the product mixture of step (A) will usually contain unreacted hydrogen fluoride in addition to 1,1,1,2-tetrafluoroethane, hydrogen chloride and by-products. It may also contain unreacted 1-chloro-2,2,2-trifluoroethane. Preferred reaction temperatures for step (A) are in the range of from 250 to 500° C., more preferably in the range of from 280 to 400° C. and particularly in the range of from 300 to 350° C. Preferred reaction pressures for step (A) are in the range of from 0 to 30 barg, more preferably in the range of from 10 to 20 barg, for example around 15 barg. Preferred residence times in the first reaction zone are in the range of from 1 to 600 seconds, more preferably in the range of from 1 to 300 seconds and particularly in the range of from 1 to 100 seconds.

In step (B), usually from 10 to 50 moles of hydrogen fluoride and preferably from 12 to 30 moles of hydrogen fluoride per mole of trichloroethylene are employed. Again, the reaction product of this stage will normally contain unreacted hydrogen fluoride and may also contain unreacted trichloroethylene. Preferred reaction temperatures for step (B) are in the range of from 180 to 300° C., more preferably in the range of from 200 to 300° C. and particularly in the range of from 220 to 280° C. Preferred reaction pressures for step (B) are in the range of from 0 to 30 barg, more preferably in the range of from 10 to 20 barg, for example around 15 barg. Preferred residence times in the first reaction zone are in the range of from 1 to 600 seconds, more preferably in the range of from 1 to 300 seconds and particularly in the range of from 1 to 100 seconds.

Although the reactant mixtures that are conveyed to the first and second reaction zones must include hydrogen fluoride, this does not mean that a fresh or virgin supply of material has to be delivered to both reaction zones. For example, the process can be operated so that virgin hydrogen fluoride is only introduced into the second reaction zone in sufficient excess that enough unreacted hydrogen fluoride can be recovered from the product mixture exiting step (B) to drive the hydrofluorination reaction that occurs in the first reaction zone in step (A). One possibility is to operate step (D) of the process so that the 1-chloro-2,2,2-trifluoroethane composition that is collected also contains hydrogen fluoride in a sufficient quantity for the reaction in the first reaction zone. Alternatively, the process can be operated so that virgin hydrogen fluoride is only introduced into the first reaction zone in sufficient excess that enough hydrogen fluoride remains in the product mixture of step (A) that is conveyed to the second reaction zone for reaction with the trichloroethylene. Additionally, after start up, the hydrogen fluoride required for the hydrofluorination reactions in the first and second reaction zones could even be introduced into a distillation column used to conduct step (D) of the process.

The reaction and separation steps which make up the preferred multi-step process for making 1,1,1,2-tetrafluoroethane may be performed using conventional equipment and techniques. Step (D), which is effectively a separation/purification step in which the useable components making up the product collected from step (B) are substantially separated from one another, may be effected by conventional distillation, phase separation and washing/scrubbing processes known to those skilled in the art.

The operation of the preferred multi-step process for making 1,1,1,2-tetrafluoroethane is described more particularly in EP-A-0449617.

In another preferred embodiment, the catalyst of the invention is used in a process for preparing pentafluoroethane which comprises reacting dichlorotrifluoroethane with hydrogen fluoride in the vapour phase at elevated temperatures in the presence of the catalyst. Reaction temperatures of at least 280° C., e.g. in the range of from 280 to 400° C. are typically employed, with reaction temperatures in the range of from 280 to 380° C. being preferred and reaction temperatures in the range of from 300 to 360° C. being especially preferred. The process may be carried out under atmospheric or super-atmospheric pressures. Typically, the process is conducted at a pressure of from 0 to 30 barg, preferably at a pressure of from 12 to 22 barg and more preferably at a pressure of from 14 to 20 barg.

In yet another preferred embodiment, the catalyst of the invention is used in a multi-step process for preparing pentafluoroethane which comprises reacting perchloroethylene with hydrogen fluoride in the presence of the catalyst to form dichlorotrifluoroethane. The dichlorotrifluoroethane is then reacted with further hydrogen fluoride in the presence of the catalyst to form the pentafluoroethane. The conversion of perchloroethylene to dichlorotrifluoroethane and the conversion of dichlorotrifluoroethane to pentafluoroethane may be conducted in discrete reaction zones of a single reactor, but they are preferably conducted in different reactors. Both reactions are conducted at elevated temperatures in the vapour phase.

The preferred pressure and temperature conditions for the conversion of dichlorotrifluoroethane to pentafluoroethane are as specified above.

For the conversion of perchloroethylene to dichlorotrifluoroethane, the process is typically conducted at a temperature in the range of from 200 to 350° C., preferably in the range of from 230 to 330° C. and particularly in the range of from 240 to 310° C. Atmospheric or super-atmospheric pressures may be employed in the process. Typically, the process is conducted at a pressure in the range of from 0 to 30 barg, preferably at a pressure in the range of from 10 to 20 barg and more preferably at a pressure in the range of from 12 to 18 barg.

A particularly preferred embodiment of the above-described multi-step process for to preparing pentafluoroethane from perchloroethylene comprises the steps of:

-   (A) in a first reactor or a first plurality of reactors reacting     perchloroethylene with hydrogen fluoride in the vapour phase at a     temperature of from 200 to 350° C. in the presence of a     chromium-containing fluorination catalyst of the invention to     produce a composition comprising dichlorotrifluoroethane, hydrogen     chloride, unreacted hydrogen fluoride and perchloroethylene, less     than 2 weight % of chlorotetrafluoroethane and pentafluoroethane     combined and less than 5 weight % of compounds having the formula     C₂Cl_(6−x)F_(x), where x is an integer of from 0 to 6, based on the     total weight of organic compounds in the composition; -   (B) subjecting the composition from step (A) to a separation step to     yield a purified composition comprising at least 95 weight % of     dichlorotrifluoroethane and less than 0.5 weight % of compounds     having the formula C₂Cl_(6−x)F_(x), where x is an integer of from 0     to 6, based on the total weight of organic compounds in the     composition; and -   (C) in a second reactor or a second plurality of reactors reacting     the composition from step (B) with hydrogen fluoride in the vapour     phase at a temperature of at least 280° C. in the presence of a     chromium-containing fluorination catalyst of the invention to     produce a composition comprising pentafluoroethane and less than 0.5     weight % of chloropentafluoroethane, based on the total weight of     organic compounds in the composition.

By compounds of formula C₂Cl_(6−x)F_(x), where x is from 0 to 6, we include trichlorotrifluoroethane and dichlorotetrafluoroethane.

In step (A), from 3 to 50 moles of hydrogen fluoride are usually employed per mole of perchloroethylene. Preferably, from 4 to 20 moles of hydrogen fluoride and more preferably from 4 to 10 moles of hydrogen fluoride are used per mole of perchloroethylene. Preferred reaction temperatures and pressures for step (A) are as discussed above for the conversion of perchloroethylene to dichlorotrifluoroethane. Preferred residence times for the reactants in the first reactor in step (A) are in the range of from 10 to 200 seconds, more preferably in the range of from 30 to 150 seconds and particularly in the range of from 60 to 100 seconds.

In step (C), from 2 to 20 moles of hydrogen fluoride are usually employed per mole of dichlorotrifluoroethane. Preferably, from 2 to 10 moles of hydrogen fluoride and more preferably from 2 to 6 moles of hydrogen fluoride are used per mole of dichlorotrifluoroethane. Preferred reaction temperatures and pressures for step (C) are as discussed above for the conversion of dichlorotrifluoroethane to pentafluoroethane. Preferred residence times for the reactants in the second reactor in step (C) are in the range of from 10 to 200 seconds, more preferably in the range of from 20 to 100 seconds and particularly in the range of from 30 to 60 seconds.

The reaction and separation steps which make up the preferred multi-step process for making pentafluoroethane may be performed using conventional equipment and techniques. Separation step (B) may, for example, be effected using conventional distillation, phase separation and washing/scrubbing processes known to those skilled in the art.

The operation of the preferred multi-step process for making pentafluoroethane is described more particularly in WO 2007/068962.

It is preferred to operate processes that use the catalyst of the invention continuously, except for any shut-down time that is necessary to regenerate or reactivate a catalyst that has been deactivated though use. The feeding of air to the catalyst during operation of the process may counter catalyst deactivation and reduce the frequency of process shut-downs for catalyst regeneration.

The present invention is now illustrated but not limited by the following examples.

EXAMPLE 1

Catalyst Preparation

A catalyst sample (Catalyst A) was prepared on a 1 tonne/day scale as follows.

A mixture of zinc and chromium (III) hydroxides was made by co-precipitation from an aqueous solution of zinc and chromium (III) nitrates using ammonium hydroxide (12.5% w/w ammonia in deionised water). The solution of zinc and chromium nitrates contained a chromium content of approximately 10% w/w and a zinc content of approximately 1.3% w/w to achieve a loading of zinc in the finished catalyst formulation of around 8.0 weight %. The equipment employed comprised a cooled and stirred precipitation tank which was fed with an aqueous stream comprising the zinc and chromium nitrates and a separate stream of ammonium hydroxide. The tank stirrer was rotated at 500 rpm during catalyst preparation. The mixed-nitrates feed and ammonium hydroxide feed were injected continuously into the tank at a point close to the stirrer blade to ensure rapid mixing. The mixed-hydroxide product formed in the precipitation tank was collected at an overflow point which maintained a constant slurry volume in the precipitation tank during a catalyst preparation. The vessel walls were cooled to maintain a constant temperature and the ammonium hydroxide pumping rate adjusted to maintain the pH of the slurry in the range of 7 to 8.5.

Slurry from the precipitation tank was filtered to recover the co-precipitated hydroxide mixture, which was then washed and filtered further.

Batches of washed solid were then dried at elevated temperatures overnight in a nitrogen atmosphere, crushed to a powder, mixed with 2% w/w graphite and compacted to form pellets of about 6 mm in diameter and length. The compacted pellets were then calcined at 300° C. under a flow of nitrogen.

Measurement of Zinc Distribution

The catalyst produced was sampled and the zinc distribution across the surface of the catalyst samples measured by Energy Dispersive Analysis of X-rays (EDAX) using a Transmission Electron Microscope.

The zinc distribution for a typical sample of Catalyst A is shown in FIG. 1.

Catalyst Testing:

Catalyst A was used to catalyse the reaction of 1-chloro-2,2,2-trifluoroethane with HF to produce 1,1,1,2-tetrafluoroethane and its stability investigated by measuring the decrease in catalytic activity with use. The catalyst test rig comprised a single Inconel reactor tube.

6 g of catalyst pellets reduced to a particle size of from 2.0 to 3.35 mm was loaded into the Inconel reactor tube and dried at 250° C. and 3 barg pressure under flowing nitrogen (80 ml/min) for 16 hours. The catalyst was then fluorinated using a mixture of nitrogen (80 ml/min) and HF (4 ml/min) at around 3 barg pressure. The fluorination process was commenced at 300° C. and the reactor temperature was then ramped to 380° C. at 25° C./hr. The temperature was maintained at 380° C. for a further 7 hours.

After the 7 hours had elapsed, the nitrogen flow was stopped and the reactor was cooled to 315° C. with the HF flow still running. 1-chloro-2,2,2-trifluoroethane (30 ml/min) and HF (90 ml/min) were then fed to the reactor at a pressure of 14 barg.

The temperature required to achieve a 10% conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane was determined and recorded. The temperature at which the 10% conversion occurred was taken to be a measure of catalyst activity—a more active catalyst allows the 10% conversion to be achieved at a lower reaction temperature.

After 48 hours of operation, the feed of 1-chloro-2,2,2-trifluoroethane was stopped and the catalyst subjected to a regeneration cycle which involved passing a mixture of HF (90 ml/min) and air (6 ml/min) through the reactor at a pressure of 14 barg while maintaining the reactor at a temperature of 380° C. This process was continued for at least 40 hours.

After completing the regeneration process, the flow of air was stopped and the reactor cooled to 315° C. with the HF flow still running. The flows of HF and 1-chloro-2,2,2-trifluoroethane were then recommenced and the activity of the catalyst investigated in exactly the same manner as before. After the end of another 48 hour operating cycle, the catalyst was regenerated, once again, in the same manner as before.

The operating and regeneration cycles were repeated several times in order to determine how the activity of the catalyst decreases with use and so obtain a measure of catalyst stability. Over the course of many operating and regeneration cycles, the catalyst loses activity and the temperature required to achieve a 10% conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane gradually increases. The increase in reactor temperature that is needed to continue to achieve the 10% conversion is a measure of catalyst stability, as more stable catalysts are able to resist the cyclic thermochemical stressing better, resulting in smaller increases in the temperature that is needed to achieve the 10% conversion.

The performance of Catalyst A is shown in FIG. 3.

EXAMPLE 2

Catalyst Preparation

A catalyst sample (Catalyst B) was prepared on a 1 tonne scale using the same procedure as described above for preparing Catalyst A except that a static mixer was placed in the chromium nitrate/zinc nitrate feed line and the combined feed was ejected into the precipitation tank using a jet mixer in order to improve the zinc distribution in the final catalyst.

Measurement of Zinc Distribution

The catalyst produced was sampled and the zinc distribution across the surface of the catalyst determined as before. The zinc distribution for a typical sample of Catalyst B is shown in FIG. 2.

Catalyst Testing:

Catalyst B was used to catalyse the reaction of 1-chloro-2,2,2-trifluoroethane with HF and its stability investigated as described above for Catalyst A.

The performance of Catalyst B is shown in FIG. 3. It is evident that Catalyst B, with its improved zinc distribution, was more stable and better able to withstand the cyclic thermochemical stressing. 

The invention claimed is:
 1. A chromium-containing fluorination catalyst which comprises from 0.5 to 25% by weight of zinc based on the total weight of the catalyst, said zinc being contained in aggregates which have a size across their largest dimension of up to 1 micron and which are distributed throughout at least the surface region of the catalyst and wherein greater than 40 weight % of the zinc-containing aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in the zinc-containing aggregates and, wherein the catalyst is amorphous or partially crystalline containing from 0.4 to 5% by weight of one or more crystalline compounds of chromium and/or one or more crystalline compounds of zinc.
 2. A chromium-containing fluorination catalyst which comprises from 0.5 to 25% by weight of zinc based on the total weight of the catalyst, said zinc being contained in aggregates which have a size across their largest dimension of up to 1 micron and which are distributed throughout at least the surface region of the catalyst, wherein greater than 60 weight % of the zinc-containing aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in the zinc-containing aggregates and wherein the catalyst is amorphous or partially crystalline containing from 0.1 to 50% by weight of one or more crystalline compounds of chromium and/or one or more crystalline compounds of zinc.
 3. The catalyst of claim 2, wherein greater than 80 weight % of the zinc-containing aggregates contain a concentration of zinc that is within ±2 weight % of the modal concentration of zinc in the zinc-containing aggregates.
 4. The catalyst of claim 2, wherein greater than 90 weight % of the zinc-containing aggregates contain a concentration of zinc that is within ±2 weight % of the modal concentration of zinc in the zinc-containing aggregates.
 5. The catalyst of claim 2, wherein the number of zinc-containing aggregates in each square millimeter of the catalyst surface or in each square millimeter of the catalyst bulk is within ±2 % of the mean number of zinc-containing aggregates per square millimeter of the catalyst surface or catalyst bulk.
 6. The catalyst of claim 2, wherein the zinc-containing aggregates have a size in the range of from 20 nm to 1 micron.
 7. The catalyst of claim 2 wherein the catalyst has been subjected to a calcination process and then subsequently fluorinated.
 8. The catalyst of claim 2, wherein from 92.0 to 100% by weight of the chromium in the catalyst based on the total weight of said chromium is present as chromium (III).
 9. The catalyst of claim 2, further comprising one or more chromium (VI) compounds and wherein from 0.5 to 4.0 % by weight of the chromium in the catalyst based on the total weight of said chromium is present as chromium (VI).
 10. The catalyst of claim 9, wherein from 96.0 to 99.5% by weight of the chromium in the catalyst based on the total weight of said chromium is present as chromium (III).
 11. The catalyst of claim 2, wherein the zinc is present in the catalyst in an amount of from 1.0 to 6.0% by weight based on the total weight of the catalyst.
 12. The catalyst of claim 2, wherein the surface area of the catalyst is in the range of from 20 to 300 m²/g.
 13. The catalyst of claim 2, wherein the surface area of the catalyst is in the range of from 100 to 250 m²/g.
 14. The catalyst of claim 2, wherein the catalyst comprises one or more chromium (III) compounds selected from the group consisting of chromia, chromium (III) fluoride, fluorinated chromia and chromium (III) oxyfluorides.
 15. The catalyst of claim 2, wherein the catalyst contains from 0.4 to 5% by weight of one or more crystalline compounds of chromium and/or one or more crystalline compounds of zinc. 